ERP5TypeTestCase: support using pinDateTime as a context manager

exiting the context manager restores the original DateTime behavior
5 jobs for for_testrunner_1
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

failed ERP5.UnitTest-Master

failed ERP5.UnitTest-Master.Medusa

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.Eggs.UnitTest-Master.Python3